Return filename for image field
Return filename for image field

Return the filename of the image form field instead of ``undefined``
when the File API is not available.  This lets us differentiate between
the case where no value has been specified for the field and when the
File API isn't supported.

Addresses #595

denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase